I've noticed over the past few months that the motor noise seems to change at Waterloo. West of waterloo it's the normal relatively quiet sound when at speed, East of Waterloo it's quite a loud electrical whistling sound similar to being on a Pendolino. Was it always thus or has something changed this year?
Alex, yes agreed, I noticed it change about two weeks ago and it's now really clear between Green Park and Bond Street. As the train leaves Bond Street you can hear a real growl too. I still have no idea what it is though.
It may just be the railheads. I know that they did some railhead grinding on the Wimbledon branch a while back and the trains sounded noticeably different.
Indeed, last weekend the whole District Line from Whitechapel to Upminster had the grinding machines working both tracks, so the track noise distinctly changed on Monday.
The noise from the District line after this weekend is remarkably loud and noticeable now along this section, you can hear the trains passing at quite a distance. I can imagine it is very loud in a tunnel if the same is done on the jubilee.
I'm certain it is rail grinding. It appears to have been done between Baker Street and Southwark in the last few days. It sounds like a loud version of the old DC motors and can clearly be heard in the trailer cars too, and disappears once away from that section of line, which is why I'm sure it is rail grinding and not something done to the motors.